Taxonomy & naming
Sternarchorhynchus mormyrus was described by Franz Steindachner in 1868 and belongs to the family Apteronotidae — the ghost knifefishes — within the order Gymnotiformes, the New-World electric knifefishes. The genus Sternarchorhynchus is one of several "tube-snout" lineages within the family, distinguished by an elongated, tubular snout that gives the group its common nickname. As with other Apteronotidae, the electric organ is built from modified nerve tissue rather than muscle, which is what lets these fish generate the fastest, highest-frequency discharges of any knifefish group.
The genus name nods to the resemblance between its long snout and that of the African elephantnose fishes (Mormyridae) — a resemblance that is purely convergent, not a sign of relationship; Sternarchorhynchus and the mormyrids evolved their tube snouts independently, on different continents, in unrelated lineages that both happen to rely on weak electric fields to navigate murky water. The species is likewise unrelated to the Asian featherback and clown knifefishes (Chitala, Notopterus), which share only the common name "knifefish" and the superficial blade-shaped body. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name.
Morphology
Sternarchorhynchus mormyrus has the elongated, laterally compressed, blade-like body typical of the ghost knifefishes, tapering to a fine tail with no proper caudal fin and no dorsal fin at all. Its most distinctive feature is the snout: long, narrow and tubular, curving gently downward, an adaptation for probing into crevices, leaf litter and fine sediment where other fish cannot reach. Propulsion comes almost entirely from a long anal fin running beneath the body, undulated in travelling waves that let the fish move forward or backward with equal ease.
Like other Apteronotidae it is scaleless, with an electric organ running down the body and electroreceptive pores scattered across the skin that read the field the organ produces. Documented size is sparse for this species; based on related tube-snout Sternarchorhynchus it is a modestly sized knifefish, well short of the black ghost's near-half-metre bulk. Coloration is typically a plain, unremarkable brown to olive, consistent with a fish that relies on electrolocation rather than eyesight and has little use for visual display.
Habitat
Sternarchorhynchus mormyrus is known from the Amazon basin, where it is associated with the flowing channels and tannin-stained blackwater tributaries typical of the genus. Data on its precise range and habitat preferences are sparse — this is not a well-surveyed species, and much of what is inferred about its ecology is extrapolated from better-studied relatives in the same tube-snout genus.
Water in this part of its range is warm, generally soft and acidic to near-neutral, and often deeply stained by dissolved tannins from leaf litter and wood — conditions in which sight is of limited use and an electric sense is a considerable advantage. Like other ghost knifefishes it is presumed nocturnal, sheltering by day among roots, submerged wood and leaf litter and becoming active after dark, though direct behavioural observation of this particular species is limited.
Feeding
As a tube-snout ghost knifefish, Sternarchorhynchus mormyrus is presumed to feed on small benthic invertebrates — aquatic insect larvae, worms and similar prey hidden in sediment, leaf litter and crevices — located by electrolocation rather than sight. The elongated, curved snout is thought to be an adaptation for reaching into narrow spaces that a shorter-snouted knifefish could not probe, a specialisation shared with the other Sternarchorhynchus species.
Specific dietary data for this species has not been documented in the sources available; the above is inferred from its morphology and from the general feeding ecology of the genus and family. Nothing is recorded about its behaviour in aquaria, since it is essentially unknown in the hobby.
Mating
No species-specific courtship or spawning behaviour has been documented for Sternarchorhynchus mormyrus. In the wave-type Apteronotidae generally, the electric organ discharge doubles as a social signal — carrying information about a fish's identity, sex and readiness to spawn — and courtship in the family is understood, from work on better-studied relatives such as the brown ghost, to involve shifts in discharge frequency between potential mates, an electrical exchange layered over physical approach in the dark.
Whether S. mormyrus follows this same pattern is a reasonable inference from its family membership rather than a directly observed fact. Data on this species' reproductive behaviour is sparse to the point of being effectively unrecorded.
Breeding
Sternarchorhynchus mormyrus is presumed to be an egg-layer, consistent with the rest of the Apteronotidae, but no breeding has been documented in aquaria and the species is not established in the hobby trade. Like most tube-snout ghost knifefish, it is a deep-channel Amazonian fish rarely collected or exported, and essentially nothing is recorded about its spawning behaviour, cues or any parental care.
For the aquarist the honest position is that this is an unbred, poorly documented species: there is no protocol to describe, and none should be inferred. Any account of its reproduction beyond family-level generalities about egg-laying and electric courtship signalling would be speculation rather than sourced fact.
In the aquarium
Sternarchorhynchus mormyrus is not an established aquarium fish; it is essentially unavailable in the trade and almost nothing has been published on keeping it. What can be said follows from its family and from general Apteronotidae care: it would need warm, soft, near-neutral to acidic water with strong, mature filtration, since like all ghost knifefishes it is scaleless and correspondingly sensitive to ammonia, nitrate and to copper-based or otherwise harsh medications, which should be avoided or heavily reduced.
As a presumed nocturnal, electrolocating predator it would need dim lighting and secure cover — root tangles, tubes or similar hides — to feel settled by day, and would not be a safe companion for small, bite-size fish or shrimp that it could hunt after dark. Anyone encountering this species in the trade should treat it as a specialist, poorly documented fish rather than a routine community animal, and should favour any available information on wild collection locality to try to replicate its native water conditions.
Conservation
Sternarchorhynchus mormyrus is assessed on the IUCN Red List as Least Concern. Plausible general pressures on its Amazon basin habitat include deforestation, water pollution and damming, but none of these have been tied to this species specifically.
Much of this genus remains under-surveyed relative to its diversity, so this Least Concern listing should be read as reflecting a lack of evidence for decline rather than a dense body of population research specific to S. mormyrus.